No matter the time of year, plan on getting wet. There's a reason Ireland is green all the time. I've been there when there has been little rain and when it's always rained. Its a chance you take just like most vacations. Generally speaking the weather is a little more temperate than Minnesota. Not as warm, but not as cold either. Having said that, the best time of the year for weather and least chance of rain is usually August or September. September is at the end of the peak season for travel in Ireland so depending on where you're flying from and to, you might be a little more limited in flight options.